Theoretical Studies On Electrostatic Stark Deceleration For Ammonia Molecules

Electrostatic Stark deceleration has become an important means for manipulating and producing cold molecular samples since its first experimental realization in 1999. Until now many polar molecular samples has been successfully decelerated based on various types of decelerator which is invented by multiple research groups. It is very difficult to realize efficient deceleration and cooling for some pole molecular samples which have relatively small electric dipole moment just like NH3 molecule. Therefore, we theoretically investigate the efficient deceleration of subsonic NH3 molecular beams based on our second-generation electrostatic Stark decelerator with 180 stages.Firstly, we introduce the calculation of the Stark splitting of polar molecules by using perturbation theory and matrix diagonalization and give some computational results of multiple types of polar molecules. By calculating the Stark splitting, we can clearly know which pole molecules in which quantum states are suitable for electrostatic Stark deceleration. Secondly, the principle of Stark deceleration is expounded. We introduce the deceleration process from longitudinal movement and transverse movement. Finally, we theoretically study the slowing effects of NH3 molecular beams by using several modes of slowing based on our second-generation electrostatic Stark decelerator with 180 stages. Our conclusions are as follows:a) we find that a subsonic NH3 beam can be successfully decelerated to 6.7m/s by using the traditional mode of slowing, removing 99.9%of their translational kinetic energy, and a molecular packet with the temperature of 80mK can be obtained; b) in order to further reduce the energy spread of the molecular pockets, we study another two modes of slowing. In one mode of slowing, the incident molecular beam is first bunched using an appropriate number of stages and then slowed down to the desired velocity at a high phase angle using the remaining stages. Our result shows that a molecular packet can be slowed from 280m/s to 20.7m/s, corresponding to a decrease of temperature from 1.34K to 1.6mK. In another mode of slowing, a high phase agnle is chosen to aggressively slow the molecules using the first some stage. The remaining stages are then operated at 0° phase angle to bunch the molecular pockets. It found that a molecular packet can be decelerated from 280m/s to 21.5m/s, corresponding to a decrease of temperature from 1.34K to 700uK. By the results obtained from the three operation modes, we found that more molecular number can be botain in the traditional mode of slowing which is operated at low phase angle and a narrow energy spread molecular pocket with a temperature of 700uK can be obtain in the third mode,which is an indubitable advantage for molecular scattering studies. However, this gain in energy resolution comes at the expense of losing molecules especially at low final velocity, which is disadvantage for the scattering experiment due to insufficient collision numbers. So, the second mode is complementary for the third mode. This shows that our 180-stage Stark decelerator can be used to realize an effective slowing and cooling for molecular samples, which have relatively small electric dipole moment just like NH3. As a result, we obtain a slow and cold NH3 molecular packet with a temperature of about 1mK, which will provide a reliable theoretical basis for our further experimental research.